This invention relates to the art of RF transmission employed in RF broadcast transmitting systems and, more particularly, to providing a radio frequency (RF) expansion joint for electrically coupling a pair of axially spaced conductor sections together as the sections axially expand and contract due to temperature variations.
The invention is particularly described in conjunction with providing an expansion joint for adjacent sections of an inner conductor of a coaxial transmitting antenna. It is to be appreciated, however, that the invention may also be employed for use with coaxial transmission lines of the type employed in a broadcast antenna feed system.
An RF transmission system includes a relatively small diameter inner conductor, which may be made of copper, and which is telescoped into a substantially larger diameter outer conductor, which may be made of copper, aluminum or steel. Whether the inner and outer conductors are of the same or different materials, it is apparent that when high power is applied the relatively small diameter inner conductor will thermally expand and contract in an axial direction to a greater degree than that of the outer conductor. It is desirable to compensate for the differential expansion between the inner and outer conductors in such an RF transmission system.
It has been common in the prior art to employ a watch band spring expansion joint to connect adjacent coaxially spaced sections of the inner conductor. Such a watch band spring joint typically includes a bullet member mounted to one of the conductor sections and a watch band spring device mounted to the other conductor section to provide a telescopic sliding fit therebetween to permit relative expansion and contraction. However, repeated sliding of the components against one another results in severe mechanical wear. It is known that such -watch band spring connections are rather fragile and prone to problems that may arise from improper manufacture, improper assembly and excessive wear. An example of such a watch band spring joint takes the form of that disclosed in the U.S. Pat. No. to G. W. Ziegler, Jr. 3,245,027.
Another prior art device takes the form of that disclosed in the U.S. Pat. No. to W. W. Seal et al. 4,543,548. That patent discloses an expandable and contractible bellows made of conductive material which is connected between opposing ends of adjacent inner conductor sections. The bellows surrounds a mechanical supporting structure including an elongated tube having an end fixed to one of the conductor sections. The opposite end of the tube is slidably received within a sleeve which is connected to the other conductor section. This tube and sleeve construction provides mechanical support for the bellows to prevent the bellows from being overstressed in tension, compression and/or torsion during use. However, such a bellows will, with use over tee, result in metal fatigue causing tiny breaks in the bellows which may result in arcing and burn out.